This invention relates to the field of display systems and more particularly to backlighted liquid crystal display systems.
Displays which are capable of forming images of characters or patterns may be broadly broken down into two different categories, active and passive. In active displays, such as light emitting diode (LED) displays, the images are composed of individual diodes which emit their own light. Since active displays are typically characterized by high power consumption, the choice for low power applications, such as portable calculators, watches, portable radios, and pocket pagers, is typically the passive display. An example of a passive display is the liquid crystal display (LCD). Rather than emitting their own light, LCD images merely reflect or absorb light, therefore, ambient sun light or room light is normally required to view the display. When the ambient light intensity is not sufficient to illuminate the display, however, an internal supplemental illumination means is typically provided. In a simple supplemental illumination system one or more light sources, typically incadescent lamps, are placed behind or in front of the display. One of the disadvantages of the simple supplemental illumination system is the creation of "hot spots". "Hot spots" are areas of the display where the light intensity is considerably greater than in other areas. "Hot spots" result in poor display readability. To correct the problem of "hot spots" and to more evenly distribute the light coming from the light sources, a light guide may be positioned behind the liquid crystal display.
A prior art light guide is illustrated in FIGS. 1 and 2. The light guide is normally made from a slab of transparent plastic material generally designated as 10 and has an upper surface 12, a lower surface 14, and four bordering surfaces 16, 18, 20 and 22. Light sources 24 and 26 are positioned respectively in notches 28 and 30 which are located at opposite ends of the slab 10. The bottom surface 14 has two planes 32 and 34 depressed into the bottom surface, thereby forming a V-shaped wedge. The top surface 12 has two areas 36 and 38 which are covered with a reflective coating. A reflective coating also covers the bottom surface 14, the depressed planes 32 and 34, and the bordering surfaces 16, 18, 20 and 22 with the exception of notches 28 and 30 which remain transparent.
In operation, light emitted from the light sources 24 and 26 travels the length of the light guide towards the planes 32 and 34, strikes the reflective surface thereon, and is reflected up through the transparent surface 12. The liquid crystal display (not shown) is situated above front surface 12 and the light passing through the front surface 12 also passes through the liquid crystal display, thereby improving display visibility.
Although this prior art light guide more evenly distributes light across the display then the simple supplemental illumination system, it also has several disadvantages, one of which is its thickness. Thinness is extremely important in the design of small watches, pocket calculators, pocket pagers, portable radios and any other device in which small packaging size is paramount. An acceptable design of a 1.8 inch long prior art guide may only be made as thin as 0.070 inches as measured at the thickest point. Any attempt to make the prior art light guide any thinner, creates an unacceptable distribution of light which results in a wide discrepancy between the intensity of the light emitted at one point on the front surface 12 and another point.
The problem is illustrated graphically in FIG. 7, wherein the vertical axis 50 plots the normalized intensity of light emitted from the front surface 12. The horizontal axis 52 plots the position (that the intensity is measured at) along the sectional line 2--2 (FIG. 1). Point 54 on horizontal axis 52 indicates the position of the left-most light source 24, point 56 indicates the position of the right most light source 26, and point 58 is a position equidistant between the two light sources. The graph is normalized, such that the intensity of position 54 and 56 is 1.0 (both sources are assumed to be of equal intensity).
The curve indicated by dotted line 60 represents the graph of a thin prior art light guide showing its distribution of light intensity across the width of the light guide. Specifically, the dip at point 62, being 70% below the intensity at points 64 and 66, is unacceptable to the average viewer. Curve 60, also illustrates another disadvantage associated with thin prior art light guides, the problem of "hot spots". Hot spots are indicated at points 68 and 70 on curve 60 and are characterized graphically as peaks in the curve.
Generally, prior art light guides are characterized by light transmission inefficiency. To compensate for this, incadescent lamps are used as light sources because they emit more intense light than other light sources of equivalent physical size. The use of the higher intensity incadescent lamps, however, requires additional power consumption, a disadvantage in electronic equipment generally, but a particular difficulty in battery operated equipment where power consumption is critical.
Furthermore, when using liquid crystal displays special problems arise when attempting to view the display outdoors at dusk or dawn. At these times, atmospheric conditions cause a shift in the ambient solar light spectrum towards the red. Because of the disportionately greater amount of red light in the atmosphere at these times, it would be advantageous to have a light source that had a spectral output at a wavelength considerably shorter than that of red light. Incadescent bulbs, with their characteristic white light output, are therefore not well suited for dusk or dawn visibility. Filtering an incadescent bulb would not provide a solution, because filtering merely selectively eliminates a large portion of the visible light spectrum, instead of increasing the intensity of light at the desired wavelength.